Hastalloy C-276 is a corrosion resistance Nickel-based superalloy, The main alloying elements are Cr, Mo, Fe and W. The alloy designed is based on solid solution strengthening, and no precipitation hardening mechanism so, the alloy C-276 are not hardenable by Martensitic Structure thereby eliminating any chance of martensite formation in HAZ due to entirely austenitic structure up to the end of solidification, So unlike C-Mn or Low alloy steel, you need not worry about controlling pre-heat but you are suggested to maintain the inter-pass temperature below 150° C during multi-pass welding.
Alloy C-276 is generally supplied in the solution annealed condition. Solution annealing would dissolve all alloying elements in the austenitic matrix. Standard solution annealing treatment is comprised of soaking at 1120°C followed by rapid quenching to room temperature to prevent any re-precipitation of carbides and/or embrittling phases. Alloy C-276 is weldable, but there are two alloying elements that are of main concern, Mo and W.
The welding metallurgy of alloy C-276 becomes upset by the development of some deleterious phases, known as µ and P Phases, Both μ and P phases belong to the group of topologically close-packed (TCP) phases. These TCP phases in the fusion zone during welding has adverse effects on Tensile strength, Impact toughness and corrosion resistance. Besides this, at the end of solidification, there is also a risk of hot cracking due to segregation tendencies of elements like W and Mo present in the weld metal composition. The TCP phases P and μ are both rich in Mo and W, i.e., these elements preferentially partition into P and μ phases, depleting the Nickel-based gamma matrix of these elements. Many researchers [1-4] have reported the phenomena of Segregation tendencies of Mo and/or W differentially in dendrite core region and dendrite arm spacing with optical microscopy techniques and using SEM-EDAX Techniques in subgrain boundary and sub-grain interior. The higher the level at which Mo and W are present in the Ni-Cr-Mo-based alloy, the higher is the possibility for the formation of these brittle phases. Unfortunately, the alloy C-276 contains approximately, 15 wt% Mo in base metal composition.
So, you are welding autogenously (without filler metal) and using filler metal, in both cases, Pulse GTAW Technique is highly recommended. The Pulse current has a better advantage of low heat input coupled with a faster cooling rate that better help the distribution of the elements into the matrix. at the same time, help suppress the microsegregation and avoid the formation of TCP Phases. Pulse Current also helps refine the microstructure so you will get better Mechanical properties, particularly toughness and corrosion resistance in the welded joint.
If you are joining it with filler metal, then your choice for filler metal becomes very important. Many works of the literature (In reference) suggest using higher Mo content filler metal than base metal composition. Among them, ERNiCrMo-4 (with 17%.Wt Mo approximately), ERNiCrMo-10 (with 13%.Wt Mo approximately), covered under A5.14/A5.14M:2018 Specification For Nickel And Nickel-Alloy Bare Welding Electrodes And Rods are the best choices this is because high Mo from the filler metal will supplement any deficiency of less Mo in dendrite arm spacing than dendrite cores So the weld metal will not any face corrosion issues during application in seawater ( Naval / Marine application).
Although, ERNiCrMo-3, under matching ( with 9 % Wt. approximately is also a tempting choice but there are two problems using this filler metal. one is less Mo and the second is the presence of Nb ( 3.5 %.Wt. approximately) in filler metal composition. The Nb is having a tendency for segregation and thereby causing hot cracking in the weld joints.
I suggest you refer to the literature, Investigation by By Manikandan et al in 2015 [3-4]. In their experimental work, they have successfully welded with the following Pulse Current parameters and obtained better weld metal mechanical and chemical properties as compared to normal GTAW. They have worked with 4mm thick Hastelloy C-276 material, welded with 2.4 mm Tungsten electrode using Pulse GTAW Technique, shielded by 15 lit/min pure Argon as shielding gas.
Pulse Current (A) 165
Background Current (A) 77
% on time 60
Pulse frequency (Hz) 5
Depending on your job thickness, you may optimise and establish suitable welding process parameters. Secondly, for your Tube-To-Tube welding process, you also need to back purging to protect the root side of the weld joint from oxidation during welding with an inert shielding gas (usually welding grade argon). Refer to reference [7] for the procedure.
For procedure qualification run, in addition to Mechanical test, I advise you, Pitting or Crevice Corrosion Test (ASTM G48 Method A or B), or as required by your client.
We hope this information is useful to you!
Welding Consultant,
Weld Met Advisory Services
References:-
[1] J.N. Dupont, S.W. Banovic, A.R. Marder, Microstructural evolution and weldability of dissimilar welds between super austenitic steel and nickel-based alloys, Welding Journal, 2003, 82, 125s-135s.
[2] M.J. Cieslak, T.J. Headley, A.D. Romig, “The welding metallurgy of Hastelloy alloys C4, C22, C276,” Jr. Met. Trans A, Vol. 17A, pp. 2035-2047, 1986.
[3] Manikandan M et al, Welding Metallurgy of Corrosion-Resistant Superalloy C-276 http://dx.doi.org/10.5772/61104
[4] Manikandan M et al, Microsegregation Studies on Pulsed Current Gas Tungsten Arc Welding of Alloy C-276, International Journal of Scientific & Engineering Research, Volume 6, Issue 12, December-2015 33 ISSN 2229-5518
[5] Sumitra Sharma et al 2018, Influence of filler material composition on the microstructural evolution,
mechanical properties and corrosion behaviour of nickel-based superalloy C-276Mater. Res. Express- https://doi.org/10.1088/2053-1591/aaf9c4.
[6] A5.14/A5.14M:2018 Specification For Nickel And Nickel-Alloy Bare Welding Electrodes And Rods
[7] http://www.haynesintl.com/es/tech-briefs/general-haynes-international-information/back-purging-and-welding-requirements-for-fabrication-of-hastelloy-alloy-pipe-systems
